1. Field of the Invention
This invention generally relates to image processing, and, in particular, to an analog-to-digital image conversion technique. More specifically, it relates to a system for converting an analog image signal obtained by scanning an object or scene into a two-valued image or video signal.
2. Description of the Prior Art
In a facsimile machine and the like, an object or scene is scanned by an image sensor to obtain an analog image or video signal and then it is converted into a two-valued video signal prior to further processing such as compression and modulation. In order to obtain such a two-valued video signal, the technique called thresholding or threshold processing is commonly applied; however, in accordance with a simple threshold processing, some delicate information in the original image will be lost. For example, narrow lines in the original image will disappear, or fine portions to be represented as "black" areas are shown as "white" areas instead, and narrow gaps or spacings between lines will disappear, or fine portions to be represented as "white" areas are shown as "black" areas. One of the reasons for this is that since a sampling spot used by an image sensor for scanning an original image is finite in size, the video signal does not change abruptly or stepwise at the boundary between the black and white portions in the original image so that the wave shape of the resulting video signal becomes rather dull.
For example, when the image pattern shown in FIG. 1(A) is scanned along the straight line 1, the resulting video signal will have the wave shape as indicated in FIG. 1 (B). If this video signal is processed by application of thresholding at level T.sub.M, the portion 1' of the video signal which corresponds to the narrow line 1 in the original image and the portion 2' of the video signal which corresponds to the narrow gap 2 in the original image will disappear, so that the resulting two-valued video signal will take the form as shown in FIG. 1(C) in which the portions 1" and 2" have false image information.
In order to cope with such a problem, a typical prior art approach is to carry out the operation of emphasizing the hiqh frequency components of the analog video signal prior to the application of thresholding. However, such an emphasizing operation is not applicable to narrow lines and narrow gaps extending in parallel with the main scanning direction, or the direction of optical scanning direction, of an image sensor and it is also susceptible to noises.